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Fate of ¹⁵N-Labeled Anhydrous Ammonia under Simulated Fall and Spring Conditions¹

Many farmers have found that Fall application of anhydrous NH₃ has economic advantages over Spring application. Differences of opinion exist, however, and previous investigations at this station have shown that higher losses of N fertilizer occur when applied in Fall. Precise evaluation of the mechanisms involved are difficult in field studies, and therefore a greenhouse experiment was conducted in which the major variable was application of ¹⁵N-labeled anhydrous NH₃ before or after a freeze-thaw cycle, simulating Fall and Spring applications, respectively. Three soils were used. One, a Pella sil, was high in organic matter (4.44% organic C). The other two soils were the same soil type (Plano sil), but differed in that one was acid (pH 4.9) and the other was close to neutral (pH 6.3). Three or 4 crops of ryegrass (Lolium multiflorum) were grown, and the amount of ¹⁵N in the plants and in the soil was determined.

Only small losses of fertilizer N (<5%) occurred in the acid Plano soil that was left fallow or cropped to ryegrass. Similar N recoveries were obtained in the pH 6.3 Plano soil and the Pella soil cropped to ryegrass, but losses increased substantially in fallow pots. The magnitude of the losses were directly related to the organic matter content and initial pH of the soils. We found that freezing and thawing of soil significantly increases the rate of denitrification. Fall application of NH₃ may lead to significant fertilizer losses by denitrification, since other studies have shown that substantial nitrification will occur in Fall and early Spring.

Key Words: Nitrogen transformation • Nitrogen uptake • Nitrification • Denitrification • Ryegrass • Lolium multiflorum

² Research Assistant and Associate Professor, Department of Soil Science, respectively. Senior author is now Lecturer, Department of Agriculture, University of Melbourne, Parkville, Victoria, Australia 3052.

Received for publication May 24, 1974.